What is Singlet Oxygen? 

Singlet oxygen is the lowest excited state of the dioxygen molecule. Its lifetime in solution is in the microsecond range (3 µsec in water to about 700 µsec in C₆D₆). It undergoes several reactions with organic molecules (Ene-Reaction, Diels-Alder Reaction). These reactions have been studied for many years.

In solution, the singlet oxygen is often prepared by a process called photosensitization. A photosensitizer is irradiated to its singlet excited state, followed by conversion (called intersystem crossing) to its triplet excited state. The triplet excited sensitizer may undergo radical reactions (Type I process) or produce singlet oxygen (Type II process). Superoxide formation (another reactive oxygen species) is also possible, but rare. The type II process is usually preferred at low substrate concentration and high oxygen concentration.

During its decay back to the ground state, the singlet oxygen molecule emits some radiation in the near IR region. This luminescence is very weak, but can be detected. The Selke group uses a low-temperature GE photodiode to directly observe singlet oxygen luminescence. When a fast laser pulse (nanosecond range) is used to excite the sensitizer, the decay of the singlet oxygen molecule can be directly observed over time. A photo and a block diagram of this time-resolved set-up are below.

This time-resolved set-up allows direct determination of rate constant by which singlet oxygen is consumed. This rate constant is the sum of quenching by solvent (kd), physical quenching(kq), and reaction of substrate (kr).  kobsd = kd + [Substr.] (kq + kr)